Tensile-magnetic inspection of drill pipe and the like



3 $29,381 'lENSiLE-i/IAGNETKb INSPECTION F DRHLL PIPE AN D THE LIKE Russell C. Heidenbrand, R0. Box 178, New Iberia, La.

No Drawing. Fiied May 12, 959, Ser.

1 Claim. (Cl. 324-37) This invention relates to an improved method of inspecting elongated ferromagnetic elements for all types of defects. The present invention is particularly useful for inspecting drill pipe, but it will be understood that the method is also useful in inspecting other elongated ferromagnetic elements, such as oil field tubing and sucker rods.

As it is well known in the oil industry, most oil and gas wells of any appreciable depth are drilledby what is commonly known as the rotary method. In this method a drill bit is secured on the lower end of a series of joints of drill pipe connected in tandem relation and usually identified as the drill string. The drill string is rotated by suitable equipment located at the surface of the well bore, and drilling fluid is circulated downwardly through the drill string and then back upwardly through the annulus between the drill string and the walls of the partially formed well bore to provide a downward movement of the drill string and the formation of the well. The drill string receives numerous shocks and stresses during a drilling operation, and frequently one of the joints of drill pipe will part during a drilling operation.

When a drill pipe failure occurs, it requires an expensive and time consuming fishing job to locate that portion of the drill string lost in the well bore and requires substantial time in replacing the drill string for a continued drilling operation. It will therefore be apparent that it is desirable to anticipate drill pipe failures by inspecting each joint of the drill pipe when the drill string is periodically removed from a partially com-' pleted well bore, as when repairing or replacing a worn drill bit. However, many of the defects in a drill string which are likely to cause a failure are not visible to the naked eye. Therefore, several different inspection methods have been devised, but these methods may be divided into two major classes.

In one type of inspection method the drill pipe is placed under tension to approximately its minimum yield point. With this method the operator may determine whether or not a joint of drill pipe has sufiicient strength for re-use in a drilling operation, and if the joint can be re-used, just where it should be placed in the drill string. This method is particularly useful in locating defects affecting the tensile strength of drill pipe when the defects occur in the extreme end portions of the drill pipe, as well as in the central portion of the drill pipe, and is also attractive in that the drill pipe Will be straightened by the application of tension. I have found, however, that the tensioning of a joint of drill pipe does not always disclose some types of defects which limit the useful life of a joint of drill pipe. For example, a narrow crack in a wall of a joint of drill pipe having a minimum length may not reduce the minimum yield point of the entire joint to such an extent.

that the joint would be discarded by the operator. However, in further service of such a joint of drill pipe (when the joint is subjected to substantial shocks and stresses and substantial fluid pressure) the small crack will work its way through the entire wall of the drill pipe and cause leakage of the drilling fluid, which will in turn enlarge the crack and cause a failure.

Another, and perhaps the most popular, method of inspecting drill pipe is by magnetizing each joint of drill pipe and then measuring the variations in flux leakage 3,029,381 Patented Apr. 10, 1962 along the length of the joint. When a joint of drill pipe is magnetized and has a defect such as a crack therein, the defect ordinarily creates adjacent North and South magnetic poles which vary the leakage of magnetic flux from the drill pipe in a manner different from the remaining portion of drill pipe which is of normal construction. The variations in flux leakage along a drill pipe are measured in several different ways. One

way is to sprinkle iron filings on the outer surface of the pipe and visually observe the variation in distribution of the iron filings to note the location of any defective portions of the pipe. Another way is to move a conductor in the form of a pick-up coil through the magnetic field in or around a joint of drill pipe and register' the variations in the current induced in the conductor.

A substantial industry has been developed for-manufacturing and using different types of inspecting devices for measuring the variations in magnetic flux leakage around a magnetized joint of drill pipe. The magnetic inspection methods employed are particularly useful in locating many rather severe types of variations in the structure of a joint of drill pipe through the central portion of the joint, but such methods are not particularly useful in locating defects in the end portions of a joint of drill pipe in the vicinity of the threaded ends of the drill pipe. As it is well known, a joint of drill pipe has a male threaded portion on one end thereof (commonly known as a pin) and a female threaded portion on the other end thereof (commonly known as a box) and the pin and box are normally larger in outer diameter than the'remaining portion of the joint.

always give an indication when a portion of a joint of drill pipe has been stressed or work hardened to such to a dangerous point.

such areas.

The present invention takes advantage of the desirable features of the prior art methods and provides an inspection method by which all types of defects can be located,: whereby an operator may discard those joints or drill pipe which are likely to fail and properly grade the remaining joints of drill pipe in a drill string for properpositioning in a new drill string. In accordance with this invention each joint of drill pipe is subjected to tension for determining the'strength of the joint and for magnify? ing the variations in structure of the pipe, and then the' joint is magnetically inspected for locating the presence of invisible defects which might affect the useful lifeofthe joint. In a preferred embodiment,'-each joint is magnetized while under tension and then the variations in; flux leakage along the joint are also measured while the joint is under tension, such that the defects inthe joint will be magnified and the variations in flux leakage will be intensified, to provide the maximum .precision in measuring the variations in flux leakage.

An important object of this invention is to increase the economy of drilling oil and gas wells by the rotary method by decreasing drill pipe failures.

Another object of this invention is to properly grade both new and used drill pipe, such that the individual joints of drill pipe may be properly positioned in a drill. string to minimize the likelihood of failure of the drill string.

A further object of this invention is to locate defects in a joint of drill pipe or the like which cannot be detected by present methods.

As" a result, it has been 'diflicult to obtain an electro-rnag -T netic inspection device which will inspect both the central portion and the pin and box of a joint of drill pipe. Also, an electro-magnetic inspection method does not.

Another object of this invention is to precisely locate and identify the various defects in a joint of drill pipe or the like.

A still further object of this invention is to provide both an economical and etficient method of inspecting drill pipe and the like.

Other objects and advantages of the invention will be evident as the description proceeds.

As previously indicated the present method may be profitably used on both new and used drill pipe, although the method is primarily designed for use in inspecting drill pipe which has been used in at least one drilling operation. In accordance with this invention, each joint of drill pipe removed from a completed or partially completed well bore is first placed under tension to approximately its minimum yield strength. Any suitable apparatus may be used for placing the joint under tension, as long as the opposite end portions of the joint are pulled in opposite directions along a common axis to either straighten the pipe, or retain the opposite end portions of the pipe in axial alignment. I prefer to use an apparatus of the type illustrated in United States Patent No. 2,757,536 wherein pull pins are threaded on opposite ends of the joint and the pull pins are then forced in opposite directions along a common axis while being simultaneously moved into alignment to assure that the opposite end portions of the joint are in alignment and that the joint is straight throughout its entire length. Such apparatus is also beneficial in that the threaded portions of each joint are tested, and any defects in the extreme end portions of a joint which affect the strength of the joint will be determined.

As previously indicated, each joint is placed under tension until the minimum yield point of the joint is reached. If the minimum yield strength of the joint is below minimum specifications, the joint may be discarded. Usually, however, the minimum yield strength of a used joint or drill pipe will be above the minimum tensile strength specifications and may be re-used in some portion of a drill string. In this latter event, each joint of drill pipe can be properly graded as to tensile strength, and can be properly positioned in a drill string to assure that the joint will not be over-stressed in a further drilling operation, such that the maximum service life of the joint may be utilized.

Although the tensioning of a joint of drill pipe to its minimum yield point will permanently magnify some of the hidden defects in the joint and will facilitate a. subsequent magnetic inspection of the joint, I have found that many types of the hidden defects are greatly magnified while the joint is under tension. Therefore, in the preferred embodiment of this invention, each joint of drill pipe is magnetized while under tension to approximately its minimum yield point. The joint may be magnetized in any suitable manner, although I prefer to magnetizc the joint by passing direct current through a pair of spaced magnetizing coils positioned around the joint and connected to provide opposing magnetic poles, such that the magnetic lines of flux extend longitudinally along the joint between the coils. This direction of magnetization is particularly useful in connection with drill pipe, since the cracks in drill pipe normally extend at an angle with respect to the longitudinal axis of the pipe and thus provide prominent North and South magnetic poles at the opposite sides of the crack when the joint is magnetized in such a direction that the magnetic lines of force extend parallel with the longitudinal axis of the joint. At

this point it may be noted, however, that when the present method is used for inspecting oil field tubing, the preferable direction of magnetization is circumferentially around the pipe, since cracks in tubing normally extend parallel with the longitudinal axis of the tubing and would not provide adjacent North and South magnetic poles oricnted along the axis of the pipe. The circumferential magnetic lines of force are provided by passing direct current lengthwise through the pipe.

Any variation in the metal structure of a joint of drill pipe will cause a variation in flux leakage when the drill pipe is magnetized. As previously indicated, I have found that the flux variations are magnified when the joint is magnetized while under tension to approximately its minimum yield point, such that the various types of defects are more easily located. This magnification of the de fects is particularly true when the defect is in the form of a metal parting, such as a pin hole in the outer surface of the pipe, pitting on the inner or outer surfaces of the pipe, and/or a crack in the inner or outer surface of the pipe. The variations in flux leakage may be located and measured in various ways. For example, I may sprinkle iron filings onto the outer surface of the magnetized joint and observe the variations in distribution of the filings on the outer surface of the joint. In those portions of the joint where the metal structure is uniform, the iron filings will tend to align themself in the direction of the magnetic lines of force and will be fairly uniformly distributed over the surface of the pipe. However, in those portions of the pipe wherein the metal structure varies, the iron filings will be unevenly distributed and the defect may be easily observed. When using iron filings, a defective area may be further visually inspected to facilitate the grading of a joint.

in a preferred embodiment of this invention, I prefer to move a conductor through the magnetic field surrounding the magnetized joint under tension and register the variations in current induced in the conductor to determine both the location of defects in the joint and the extent or severity of the defects. Several different types of inspecting devices are presently on the market which utilize one or more pick-up coils supported in a suitable housing in positions to surround a joint of drill pipe, such that variable currents will be induced in the pick-up coils as the housing is moved lengthwise along a magnetized joint. Any of these inspection devices may be used in practicing the present invention. However, I prefer to move the pick-up coils generally in the direc tion of the magnetic lines of force of the magnetic field surrotmding the joint. such that a minimum current will be induced in the pick-up coils until the pick-up coils are passed through a variation in the magnetic field, to more precisely locate the defective areas and more precisely measure the severity of the defects. An apparatus of the last-mentioned type is illustrated in the co-pending application of Kenneth W. Cochran entitled Electro- Magnctic Pipe inspecting Apparatus filed April 30, 1958, Serial No. 731,971. In any event, the drill pipe may be accurately graded for re-use in a proper position in a drill string to minimize the possibly of failure of the pipe.

As previously indicated, the present method is also useful in inspecting and grading any elongated ferromagnetic elements, such as oil field tubing and sucker rods, as well as drill pipe, and similar benefits will be obtained.

From the foregoing it is apparent that the present invention will materially increase the economy of oil and gas well drilling by the rotary method by minimizing drill pipe failures. Drill pipe may be accurately graded by the present invention, such that those joints of drill pipe not meeting minimum specifications may as discarded, and all other joints inspected may be properly graded for proper positioning in a new drill string. it is also believed apparent that the present method will locate and determine the presence of defects not observable by prior methods. and that the present method is both cconomical and efiicient.

Changes may be made in the combination and arrangement of steps and procedures as heretofore set forth in the specification without departing from the spirit and scope of the invention as defined in the following claim.

5 6 I claim: References Cited in the file of this patent A method of inspecting an elongated ferromagnetic UNITED STATES PATENTS eiemerit, comprising the steps of:

(a) tensioning the element to approximately its mini- Dliake et 1933 mum yield point, 5 2,882,488 Price et a1. Apr. 14, 1959 (b) magnetizing the element, and (c) registering the variation in flux leakage along the OTHER REFERENCES iengtn of the magnetized element immediately fol- Sanford: Bureau of Standards Technical Paper No. lowing tensioning of the element. 315 (1926), vol. 20. 

